The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the acquisition of three-dimensional data sets and the reconstruction of images from such data sets.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The present invention will be described in detail with reference to a variant of the well known Fourier transform (FT) imaging technique, which is frequently referred to as "spin-warp". The spin-warp technique is discussed in an article entitled "Spin-Warp NMR Imaging and Applications to Human Whole-Body Imaging" by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980). It employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NMR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (G.sub.y) along that direction, and then a spin-echo signal is acquired in the presence of a readout magnetic field gradient (G.sub.x) in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse G.sub.y is incremented (.DELTA.G.sub.y) in the sequence of views that are acquired during the scan to produce a set of NMR data from which an entire image can be reconstructed.
In a three-dimensional implementation of the spin-warp method phase encoding of the spin-echo signals is performed along two orthogonal axes. As described in U.S. Pat. No. 4,431,968 entitled "Method of Three-Dimensional NMR Imaging Using Selective Excitation," a thick slab of spins is excited by applying a slab-selection gradient (G.sub.z) in the presence of a selective RF excitation pulse and then a first phase encoding gradient (G.sub.z) along the same axis and a second phase encoding gradient (G.sub.y) are applied before the NMR signal acquisition in the presence of a readout gradient (G.sub.x). For each value of the G.sub.z phase encoding gradient, the G.sub.y phase encoding is stepped through all its values to sample a three-dimensional region of k-space. By selectively exciting a slab, NMR signals are acquired from a controlled 3-dimensional volume.
MR angiography (MRA) has been an active area of research. Two basic techniques have been proposed and evaluated. The first class, time-of-flight (TOF) techniques, consists of methods which use the motion of the blood relative to the surrounding tissue. The most common approach is to exploit the differences in signal saturation that exist between flowing blood and stationary tissue. This is known as flow-related enhancement, but this effect is misnamed because the improvement in blood-tissue contrast is actually due to the stationary tissues experiencing many excitation pulses and becoming saturated. Flowing blood, which is moving through the excited section, is continually refreshed by spins experiencing fewer excitation pulses and is, therefore, less saturated. The result is the desired image contrast between the high-signal blood and the low-signal stationary tissues.
MR methods have also been developed that encode motion into the phase of the acquired signal as disclosed in U.S. Pat. No. Re. 32,701. These form the second class of MRA techniques and are known as phase contrast (PC) methods. Currently, most PC MRA techniques acquire two images, with each image having a different sensitivity to the same velocity component. Angiographic images are then obtained by forming either the phase or complex difference between the pair of velocity-encoded images. Phase contrast MRA techniques have been extended so that they are sensitive to velocity components in all three orthogonal directions.
When 3D imaging methods are employed to produce an MRA image, the size of the excited slab becomes a limiting factor. To improve the diagnostic utility of the MRA image it is desirable to increase the slab thickness to increase the field of view along the slab-select axis. However, time-of-flight (TOF) MRA images decrease in quality as the slab thickness increases due to the saturation of the spins as they flow through the excited slab. That is, due to the increased thickness of the excited slab, blood remains in the slab for a longer time and becomes saturated by the selective RF excitation pulse. As a result, fresh blood entering the slab appears much brighter in the reconstructed image than blood which has remained in the slab for a number of excitations.
One solution to this problem is to acquire NMR data from the desired three-dimensional region by sequentially exciting a series of thin slabs and concatenating the NMR data acquired therefrom. As described in U.S. Pat. No. 5,167,232 entitled "Magnetic Resonance Angiography By Sequential Multiple Thin Slab Three Dimensional Acquisition," the thin slabs must be overlapped because slices on each slab boundary suffer from signal loss due to imperfect slab excitation profiles. As a result, a large percentage (e.g. up to 50%) of the acquired data is discarded because of the signal fall-off at the thin slab boundaries. Without this substantial thin slab overlap and consequent reduction in acquisition efficiency, a "venetian blind" or thin slab boundary artifact (SBA) is produced. This SBA artifact is characterized by a signal loss at slab boundaries, is flow dependent, and results in a signal intensity oscillation along blood vessels, which may result in a false depiction of vessel lumen diameter and over estimation of stenosis and atherosclerosis in clinical MRA images. On the other hand, discarding half of the acquired data is inefficient and significantly increases the scan time for a specified region of interest.
Another method for reducing the signal fall-off of flowing spins as they traverse the excitation slab is to use a ramped or "TONE" RF excitation pulse to compensate for progressive signal decay (see Purdy D., Cadena G., Laub G., The Design Of Variable Tip Angle Slab Selection (TONE) Pulses For Improved 3-D MR Angiography, Book of Abstracts: Society of Magnetic Resonance in Medicine, 1992, Berlin: Germany, p. 882). However, there are a number of inherent problems with this solution. Firstly, determination of the shape of the ramped RF pulses is empirical and inaccurate due to the unpredictable flow pattern of blood in vivo. Secondly, since the ramped RF pulses are only designed to compensate for flow in one direction, the use of such RF excitation pulses leads to an exaggeration of the slab boundary artifact when blood flow reverses its direction within the imaged volume. In other words, the ramped RF pulse technique increases sensitivity to flow direction. Finally, the use of ramped RF pulses can limit flexibility in the choice of flip angle, since the optimal flip angle and ramp angles are strongly correlated (i.e. fixing one constrains the other).
Yet another solution is to use frequency modulated (FM) RF pulses to perform quadratic phase encoding as disclosed in James G Pipe, Spatial Encoding and Reconstruction in MERI with Quadratic Phase Profiles, Magnetic Resonance Imaging 33:24-333, 1995. This method provides identical weighting to flow for every resolved element in the slice-selection direction and thus removes the slab boundary effect, thereby overcoming the problems inherent in the prior art techniques. However, the quadratic phase encoding technique suffers from three disadvantages. Firstly, it requires a specialized image reconstruction method which is not simple to implement on a conventional clinical MRI scanner. Secondly, due to the low-efficiency of quadratic phase RF pulses, the specific absorption rate (SAR) is higher than with conventional methods, which restrict its application to patients according to FDA guidelines on power deposition. Finally, the signal-to-noise ratio of angiograms performed with this method is dependent on flow direction due to on-resonance and off-resonance effects.
Another method for acquiring multiple highly-overlapping 2D slices is disclosed by Juergen Hennig in "Overlapping Section Coverage in Multisection Imaging," Journal of Magnetic Resonance Imaging, 1993, March/April, pages 425-432. This technique is claimed to solve the `string-of-pearls` artifact in 2D time-of-flight MRA which is mainly caused by imperfect RF slice profiles. The key element of this method is a strategy for undersampling in the k.sub.y -axis. In this method, the reconstruction of an image at any arbitrarily defined position along the slice selection direction is attempted using data sampled at discrete slice position locations. Thus, for example, on a designated image reconstruction plane, some k.sub.y lines will come from one acquired plane and some from another acquired plane. The disadvantage of this method is that artifacts such as blurring and other partial volume effects are seen. As the author of this method proposed, the best application of this technique is in producing a factor of two improvement in effective slice direction resolution. To accomplish this, however, there should be very little phase error across the slice so that phase conjugate symmetry can be used.